Peak detection for magnetic tape timing based servo

ABSTRACT

Selection of servo signals for peak detection of timing based servo tracks of magnetic tape for lateral positioning of heads relative to the magnetic tape. Servo signals from magneto-resistive servo read head(s) reading a magnetic tape are digitized by analog to digital converter(s); digital peak measurement apparatus is configured to sense the digital converter(s), and provide positive peak values and negative peak values of the digitized servo signals; and control apparatus is configured to determine the greater of the provided positive peak values or negative peak values, and select servo signals having the greater values for a servo channel. Servo apparatus is configured to respond to the selected servo signals to control the lateral positioning of the servo read head(s).

DOCUMENT INCORPORATED BY REFERENCE

Commonly assigned U.S. Pat. No. 5,689,384, Albrecht et al., isincorporated for its showing of timing based servo systems for magnetictape.

FIELD OF THE INVENTION

This invention relates to magnetic tape timing based servo systems, and,more particularly, to detection of timing based servo signals frommagnetic tape.

BACKGROUND OF THE INVENTION

Magnetic tape media comprises a medium for storing large amounts ofdata, and typically comprises a plurality of data tracks that extendlongitudinally along the tape. A tape head is employed for readingand/or writing data on the data tracks, and is typically shared betweenvarious data tracks or groups of data tracks, and is moved betweentracks or groups of tracks in the lateral direction of the tape. Thetape head typically comprises a number of separate elements which readand/or write data with respect to a number of parallel data tracks, andis associated with one or more separate servo read heads, which arelaterally offset from the read and/or write elements, so as to trackfollow a servo band and be guided along the data track or tracks. Aservo band provides the servo guidance along any of several paths withinthe band, and the tape head is repositioned laterally within a servoband so that the read and/or write elements access different datatracks. The servo bands are continuous to provide constant trackfollowing. Incorporated U.S. Pat. No. 5,689,384 provides timing basedservo bands to allow for track following for each of a number of pathsacross the servo band.

The lateral positioning of the tape head is typically accomplished byactuators, which may have mechanical or electromechanical components.The servo read head is smaller than the lateral width of the servo band,and once the proper lateral positioning of the tape head has beenaccomplished, as the servo information being sensed by the servo readhead indicates, minor adjustments of the head to follow a particularpath within the servo band is made by lateral movement of the tape or ofthe tracks on the tape. Similarly, lateral repositioning of the tapehead to different tracks within the same servo band is accomplished byan adjustment of position within the servo band to another path.

The track following is based on the servo signal as read by the servoread head. Timing based servo systems compare the time between peaks toderive the servo read head lateral position. Therefore, the timing basedposition error signal (PES) is insensitive to speed of the magnetic tapeand insensitive to the amplitude of the servo pattern on the magnetictape. However, noise (tape or electronics) and signal dropout due tomedia defects can corrupt and cause missed peaks. Additionally,asymmetry (readback distortion), a result of reading withmagneto-resistive servo read heads, can in turn reduce peak detectionmargin, and result in missed peaks.

SUMMARY OF THE INVENTION

Selection of servo signals for peak detection of servo signals derivedfrom timing based servo tracks of magnetic tape. Servo channels ofmagnetic tape drives, magnetic tape drives, and methods and apparatussense servo signals from at least one magneto-resistive servo read headand determine the greater of the positive peak values and negative peakvalues, for selecting the positive or negative servo signals for theservo channel.

In one embodiment, the servo signals are digitized by at least oneanalog to digital converter, digital peak measurement apparatus isconfigured to sense the at least one digital converter, and providepositive peak values and negative peak values of the digitized servosignals; and control apparatus is configured to determine the greater ofthe provided positive peak values and the negative peak values, and toselect servo signals having the greater values for a servo channel.

In another embodiment, the control apparatus is configured to select theservo signals having the greater of the provided positive peak valuesand negative peak values, and servo apparatus is configured to respondto the selected servo signals to control the lateral positioning of theservo read head(s).

In a still further embodiment, the digital peak measurement apparatus isconfigured to provide the average positive peak values and averagenegative peak values; and the control apparatus is configured todetermine the greater of the provided average positive peak values andthe provided average negative peak values, for selecting servo signals.

In a still further embodiment, the control apparatus is configured todetermine the greater average peak values in accordance with theequation:

$\frac{{POSpk} + {NEGpk}}{{POSpk} - {NEGpk}}.$

In another embodiment, the control apparatus is configured to determinethe greater average peak values by comparing the positive peak valuesand the negative peak values in accordance with the equation:(POSpk+NEGpk)<0.

For a fuller understanding of the present invention, reference should bemade to the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cut away view of a data storage device, comprisinga magnetic tape drive, implementing the present invention;

FIG. 2 is a block diagram of the magnetic tape drive of FIG. 1;

FIG. 3 is a schematic representation of a tape head and a segment of amagnetic tape media with a plurality of separate servo bands inaccordance with the present invention;

FIG. 4 is a representation of a servo read head as it tracks one of theservo bands of FIG. 3, along with an idealized representation of theservo output signal it generates and the corresponding signal intervals;

FIG. 5 is an illustration of typical servo signals as read by the servoread head of FIG. 4;

FIG. 6 is a schematic and block representation of servo read heads and aservo reader in accordance with the present invention, for reading servoinformation of a plurality of separate servo bands on a magnetic tapemedia; and

FIG. 7 is a flow chart depicting a method in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is described in preferred embodiments in the followingdescription with reference to the Figures, in which like numbersrepresent the same or similar elements. While this invention isdescribed in terms of the best mode for achieving this invention'sobjectives, it will be appreciated by those skilled in the art thatvariations may be accomplished in view of these teachings withoutdeviating from the spirit or scope of the invention.

FIGS. 1 and 2 illustrate a magnetic tape drive 10 which writes data toand reads data from magnetic tape 11. As is understood by those of skillin the art, magnetic tape drives may take any of various forms. Theillustrated magnetic tape drive 10 translates the magnetic tape 11 alonga tape path in the longitudinal direction of the tape from a supply reel12 in a tape cartridge 13 to a take up reel 14, the reels comprisingdrive reels of a tape drive system which are operated by drive motors16. A single reel magnetic tape cartridge 13 is illustrated. An exampleof a magnetic tape drive 10 is the IBM 3592 TotalStorage Enterprisemagnetic tape drive. Another example of a magnetic tape drive 10 is theIBM 3580 Ultrium magnetic tape drive. An alternative magnetic tape driveand magnetic tape cartridge is a dual reel cartridge in which both reels12 and 14 are contained in the cartridge.

The magnetic tape 11 is moved in the longitudinal direction across atape head 15. The tape head may be supported and laterally moved by anactuator 17 of a track following servo system and wrap control system27. A set of roller tape guides 60, 61, 62 and 63 hold the tape inposition laterally, while motor drivers 28 operate the motors 16 to movethe tape longitudinally. The drive motors are operated at various speedsas controlled by the motor drivers 28 to insure that tape leaves onereel at the same speed that it is wound onto the other reel.

The magnetic tape head 15 may comprise a plurality of read and writeelements for respectively reading and writing data with respect to themagnetic tape 11, and may have one or more servo sensors for sensingservo tracks or bands 18 recorded on the magnetic tape. The servosensor(s) may sense the lateral position of the magnetic tape andoperate the servo system and actuator 27, 17 to track follow the servotracks or bands. The servo sensor may also detect the longitudinalposition of the tape from the servo tracks or bands. The magnetic tape11 may have a number of parallel tracks, each of which corresponds to aread and a write element of the magnetic tape head 15. The tracks may bearranged in a back and forth serpentine pattern, each set of trackscalled a “wrap”, or may be arranged as sets of bands which may be readin both directions. The tape 11 may be moved in a first (or forward)direction from the supply reel 12 to the take up reel 14 while the trackfollowing servo follows one wrap or set of tracks, and then the servoand wrap control may operate the actuator 17 to shift the magnetic tapehead laterally to another wrap, and the magnetic tape 11 moved in theopposite (or reverse) direction from the take up reel 14 to the supplyreel 12 while the track following servo follows the other wrap or set oftracks.

Also as understood by those of skill in the art, a magnetic tape drive10 comprises one or more controls 20 for operating the magnetic tapedrive in accordance with commands received from an external system 30received at an interface 21. The external system 30 may comprise anetwork, a host system, a data storage library or automation system, adata storage subsystem, etc., as is known to those of skill in the art.A control typically comprises logic and/or one or more microprocessorswith a memory 19 for storing information and program information foroperating the microprocessor(s) and drive. The program information maybe supplied to the memory via the interface 21, by an input to thecontrol 20 such as a floppy or optical disk, or by reading from amagnetic tape cartridge, or by any other suitable means. The magnetictape drive 10 may comprise a standalone unit or comprise a part of atape library or other subsystem, which may comprise the external system30.

The control 20 also provides the data flow and formatter for data to beread from and written to the magnetic tape, employing a buffer 31 and arecording channel 32, as is known to those of skill in the art.

FIGS. 3 and 4 illustrate a servo system and the magnetic tape media 11,the magnetic tape media having a plurality of separate longitudinalservo bands 41, 42, 43 and 44, which are laterally positioned on themagnetic tape media. The example of FIG. 3 comprises a magnetic tapemedia with magnetically written servo bands, and with data trackspositioned between the servo bands. The magnetic tape media is readand/or written by the tape head 15 of the magnetic tape drive. A tapehead typically comprises a number of separate elements 47, which readand/or write data with respect to a number of parallel data tracks, andis provided with a separate servo read head, or servo read heads 50, 51,which are offset from the read and/or write elements 47, so as to trackfollow the servo band and be guided along the data track or tracks.

FIG. 4 illustrates a servo read head path 55 as the servo read headtracks one of the servo bands of FIG. 3, e.g. servo band 41, along witha representation of the generated servo output signal 57 and thecorresponding signal intervals 58. The exemplary servo band is of thetiming based servo type described in the incorporated U.S. Pat. No.5,689,384, which comprises patterns of non-parallel laterally extendingtransitions recorded across the width of the servo band. The timing 58of the signal 57 derived from reading at any point across the width ofsuch a pattern varies continuously as the servo read head 50, 51 of FIG.3 is moved laterally across the servo band, since the servo read head50, 51 is small compared to the width of the servo pattern.

Lateral position sensing is achieved by deriving a ratio of two servopattern intervals “A” and “B” of FIG. 4 to allow for track following foreach of a number of paths across the servo band.

The lateral positioning of the tape head is accomplished by actuator 17,which may have mechanical or electromechanical components. The servoread head is smaller than the lateral width of the servo band, and oncethe proper lateral positioning of the tape head has been accomplished tothe correct path 55, minor adjustments of the head to follow theparticular path within the servo band is made by lateral movement of thetape or of the tracks on the tape. Similarly, lateral repositioning ofthe tape head to different tracks or wraps is accomplished by anadjustment of position within the servo band to another path, ormovement to another set of servo bands.

The track following is based on the servo signal as read by the servoread head 50, 51. Timing based servo systems compare the time betweenpeaks of the servo signal to derive the servo read head lateralposition. Therefore, the timing based position error signal (PES) isinsensitive to speed of the magnetic tape and insensitive to theamplitude of the servo pattern on the magnetic tape. However, noise(tape or electronics) and signal dropout due to media defects cancorrupt and cause missed peaks. Additionally, asymmetry (readbackdistortion), a result of reading with magneto-resistive servo readheads, can in turn reduce peak detection margin, and result in missedpeaks.

FIG. 5 illustrates two examples of asymmetric servo signals as read by amagneto-resistive servo read head. In the examples, the servo signal 70is the result of reading in a forward direction of motion of themagnetic tape, and the servo signal 71 is the result of reading in areverse direction of motion of the magnetic tape. In both, the negativepeaks are significantly longer than the positive peaks. In accordancewith the present invention, there is improved margin to determine thetiming based position error signal (PES) by working on the longest peak.

The peaks are located in time by a peak detector. These are normallythought of as the position where the second derivative is zero AND thepeak value is greater (in absolute value) than the threshold. Thetracking threshold is a separate circuit of servo control 27 thatmonitors a percent of the peak to baseline distance. Tracking thresholdsare normally implemented as envelope followers (follow max peak values).For example, a 50 percent tracking threshold would keep a level that ishalf of the peak to baseline distance, and would increase and decreasein order to keep that relationship as the peaks go up and down. Trackingthresholds are normally implemented as dual independent trackingthresholds: one for the positive signal and one for the negative side.Note that in an asymmetric waveform the two values will not beequivalent.

For timing based servo PES detection, only the distance between thepositive peaks or the distance between the negative peaks is required,as is known to those of skill in the art. The detector does not have toget both sides.

The stretched peak is better for detection for 2 reasons:

1) the taller peak provides more margin for the threshold qualification(more distance between peak and threshold and more distance betweenbaseline to tracking threshold value).

2) the stretched peak is generally sharper (more pointed) in appearance.This pointedness is good for better time positioning of the exact peak.The pointy peak is more noise tolerant for peak detection than a roundypeak. In a roundy peak, a small noise spike can result in a detectedpeak in the wrong location. This pointedness results in less peak jitterand therefore a better overall peak position estimate and therefore abetter PES.

Another potential source of a symmetry besides the magneto-resistiveservo head may comprise the high-pass filtering used to reject lowfrequency noise sources such as motor and PWM (pulse width modulation)noise, and the media asperities that cause thermal spike noise when theycollide with the magneto-resistive element itself. The high-passfiltering along with the relationship between the written servo patterndirection and the readback direction can shift the readback pattern upand down around the baseline. This can have the same effects in reducingpeak detection margins as distortion from the magneto-resistive servohead.

Referring to FIGS. 6 and 7, servo signals from at least onemagneto-resistive servo read head 50, 51 are provided and measured todetermine the greater of the positive peak values and negative peakvalues, for selecting the positive or negative servo signals for theservo channel.

In one embodiment, the servo signals are detected by servo detectors 80,81 of servo system 27, and digitized in step 83 by at least one analogto digital converter 84. Digital peak measurement apparatus 86, in step87, senses the digitized servo signals and provides positive peak valuesand negative peak values of the digitized servo signals, and may providethe average of positive peak values and the average of negative peakvalues. The average values may comprise, for example, the averagepositive peak amplitude of 65,000 positive peaks (a positive number),and the average negative peak amplitude of 65,000 negative peaks (anegative number). In step 90, control apparatus 92 employs a peakdetection algorithm, and, in step 95, determines the greater of theprovided positive peak values and negative peak values, for selectingservo signals for a servo channel.

In one embodiment, the equation:

$\frac{{POSpk} + {NEGpk}}{{POSpk} - {NEGpk}}$is employed to determine the greater average peak values in step 95, bydetermining whether the result is greater or less than 0.

If step 95 determines that the result is greater than 0, in step 97, thecontrol apparatus 92 selects the positive peaks for the servo channel100. If step 95 determines that the result is less than 0, in step 98,the control apparatus 92 selects the negative peaks for the servochannel 100.

By the above equation, asymmetry is the peak difference normalized bythe peak to peak distance. Only one number is thus maintained toindicate which peak to select. Alternatively, the tallest peak may beused directly to make the choice of step 95: if (POSpk+NEGpk)<0, choosethe negative peak in step 97, otherwise choose the positive peak. Note:NEGpk is a negative number.

Additionally, as discussed above, a tracking threshold is running, inwhich there is an independent tracking threshold for the positive andnegative side of the waveform. Thus, the above equation may be restatedto compare the thresholds, considered equivalent herein: if(POSthr+NEGthr)<0, then choose the negative peak, otherwise choose thepositive peak.

The servo channel 100 responds to the selected servo signals to controlthe lateral positioning of the servo read head(s) 50, 51 through thelateral movement of the tape head 15 by means of the actuator 17,thereby positioning the read and/or write elements 47.

Those of skill in the art will understand that changes may be made withrespect to the servo and head arrangements and that other algorithms maybe employed to determine the greater peak values. Further, those ofskill in the art will understand that differing specific componentarrangements may be employed than those illustrated herein, and thatvarious components may be combined.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the artwithout departing from the scope of the present invention as set forthin the following claims.

1. Method for selecting servo signals for peak detection of servosignals derived from timing based servo tracks of a magnetic tape, saidmethod comprising: digitizing analog servo signals derived from servotracks of a magnetic tape; sensing said digitized servo signals, andproviding positive peak values and negative peak values of saiddigitized servo signals; selecting servo signals having the greater ofsaid provided positive peak values and said provided negative peakvalues, and providing peak detected servo signals corresponding to saidselected servo signals.
 2. The method of claim 1, wherein said step ofproviding positive peak values and negative peak values comprisesproviding average said positive peak values and average said negativepeak values; and said step of selecting servo signals comprisesdetermining the greater of said provided average positive peak valuesand said provided average negative peak values, and selecting servosignals having the greater of said average peak values.
 3. The method ofclaim 1, wherein said step of selecting said servo signals comprisesdetermining said greater peak values in accordance with the equation:$\frac{{POSpk} + {NEGpk}}{{POSpk} - {NEGpk}}.$
 4. The method of claim 1,wherein said step of selecting said servo signals comprises comparingsaid positive peak values and said negative peak values in accordancewith the equation: (POSpk+NEGpk)<0.